This invention relates to submersible pumps and more particularly to pumps for pumping oil from the earth.
Prior art oil wells typically comprise a hole drilled in the ground, usually to a depth of a few hundred to several thousand feet, into which a reciprocating plunger type pump is inserted and connected to an actuating mechanism on the surface by a long, rigid rod assembly.
Once the hole is drilled, a tubular casing is installed in the hole and is cemented in place. The casing is then perforated at the depth of the formation from which the oil is to be pumped to allow oil from the formation to flow into the casing. Various additional procedures to maximize the flow rate of the oil from the formation, such as "fracing," may be performed in the casing before the pump mechanism is installed.
A typical pump mechanism comprises a stationary pump barrel with a check valve assembly in the bottom of the barrel. The top of this barrel is attached to rigid well tubing that supports the barrel in the casing at the desired depth. A tedious and time-consuming procedure is required to attach the well tubing to the barrel and lower the assembly into the casing. The tubing is provided in 30 foot lengths that are usually assembled in pairs after they are delivered to the well site. Each 60 foot long section of tubing must be lifted into a vertical position over the top opening in the casing by a winch in a work-over rig. The first section of tubing is fastened to the barrel with a threaded coupling by spinning the section of tubing. Then the tubing and barrel are lowered into the casing until just the top of the first section of tubing is projecting above the top of the casing. At this point a second 60 foot section of tubing is raised into position over the first section and is coupled to that section by a threaded coupling while the first section is held in place by a slip clutch assembly. Many hours are spent making up the well tubing and lowering it into the casing until the barrel is at the desired depth.
The second major component of the pump is the plunger assembly which comprises a polished rod that has an attached head portion of approximately the same diameter as the inside diameter of the pump barrel. A plunger ball check valve assembly is carried inside the head portion and packing about the periphery of the head portion prevents oil from flowing past it. Also, a pump rod guide is positioned on the pump rod. The plunger is attached to a pump rod which extends up through the well tubing to the surface of the earth where the pump actuating mechanism is located.
Lengths of pump rod, which usually are 20 feet long and are assembled at the well site into "triples" of 60 foot lengths, must be assembled to the plunger in the same manner as the well tubing was assembled or made up to the barrel. Each length of rod is lifted into place and attached to the previous length with threaded couplings that are integral with the ends of the rods. The final polished rod is the only rod which projects above the surface of the earth. A stuffing box and tee assembly are attached to the top of the well tubing at the wellhead, and the pipe to carry the oil to storage tanks couples to the tee. When the pump rod has been made up to the proper length, it must be set into the pump barrel. The top of the pump barrel has a seat for the pump rod guide. The pump rod guide must be "set" by dropping the full length of the rod assembly a short distance.
Once the well tubing and the rod assembly are in place, the actuating mechanism, known as a horse's head pump assembly is connected to the polished rod through a yoke and a cable. The assembly comprises a tower with a pivot on which is mounted a rocker arm with the horse's head at one end and a counter weight on the other end to counterbalance the weight of the horse's head and the rod. The rocker arm is driven by a connecting rod coupled to a crank on a gear box, which is belt driven by an electric motor. The motor provides the force to overcome friction and lift the liquid pumped as the rocker arm is raised and lowered.
The pump is commonly known as a sucker rod pump and delivers fluid on both the up and the down strokes. The diameter of the rod above the head portion is approximately 0.7 times the inside diameter of the pump barrel so that about half the oil flowing through the plunger check valve on the down stroke of the rod flows into the volume between the rod and pump barrel created above the packing on the head portion by the down stroke and the other half flows up toward the surface of the earth. On the upstroke of the rod, oil is sucked into the barrel through the barrel check valve while the column of oil in the well tubing above the plunger packing is lifted up to the surface of the earth, including that stored in the volume between the rod and pump barrel. Thus the net volumes pumped during the down and up strokes are approximately equal. However, if the head of oil in the casing outside the pump barrel is not sufficient, the pump may reach a "pumped off" condition in which dissolved gasses in the oil may be released as it is sucked through the barrel check valve during the up stroke, and part of the volume of the barrel will be filled with gas instead of liquid. In such a case, oil will not be forced into the plunger check valve during the first part of each down stroke and the column of oil above the packing will actually descend from the surface of the earth during that time. This will reduce the volumetric efficiency of the pump and can produce destructive "water-hammer" like impacts on every down stroke, thus shortening pump life.
In the prior art, the pumped off condition was usually detected by an on site person known as a "pumper" putting his hand on the polished rod where he could feel the vibration caused by the plunger hitting the top surface of the oil in the barrel. He would then adjust a timer on the pump motor so that the pump would shut off at approximately the time during each pumping cycle when the pumped off condition was reached. However, to keep a pump working at maximum efficiency requires constant attention by an experienced pumper. There are other methods of determining how long a pump should be run each day for maximum efficiency, but they usually involve even greater expense or complexity than the method just described.
One method of determining the height of the oil in the casing is to use a sound transducer to monitor sound waves reflected off the couplings when it is fired down the well by a blank firing gun. The data must be monitored and interpreted by an engineer, which is costly and can only be done infrequently. It is also possible to place pressure transducers at the bottom of the well at the pump to measure the head of oil above the pump inlet, but a wire must be run down the casing along with the well tubing to connect to the transducer. This is very time consuming, often requiring another day when the tubing string is being made up, and the wire is so delicate in comparison with the tubing and rods that the wires are often broken during makeup and must be repaired causing further time delay. One solution proposed for this problem in the prior art is shown in U.S. Pat. No. 3,434,682 entitled "Wire Positioning and Protective Device," issued 25 Mar. 1969.
Although the basic sucker rod pump is simple and reliable, it none-the-less requires periodic maintenance due to the wearing of parts, clogging of perforations in the casing or the valves by sand, paraffin or other substances, etc. When maintenance is required the well must be shut down for several days while the pump is laboriously disassembled by reversing the make up process described above. This disassembly is costly not only because of the 4 to 5 man crew required to perform the work, but because the well is not producing during the several days required to disassemble, service and reassemble the pump.
Further drawbacks to the sucker rod pump involve the efficiency of its operation, the complexity of pump selection parameters and safety. The motor driving the gear box is usually a much higher horsepower motor than would be necessary just to lift the column of oil to the surface of the earth because of the high friction and starting inertia involved in a pump that has several thousand feet of pump rod weighing thousands of pounds connected to an equally heavy rocker arm assembly. For example, pumping 30 barrels per day from a 4000 foot deep well results in a hydraulic output of one horsepower, but often a 30 horsepower motor is used. Pump selection is complicated by the fact that the tubing and rods may be resonant or nearly resonant at the frequency of the pump's oscillation, and the relative extension and contraction of the rod and tubing may be on the order of the stroke length of the rocker. Careful selection of pump stroke and rocker stroke length and frequency must be made to avoid undesirable conditions.
Safety concerns involve both workers and strangers, such as children, who may wander into an oil field. Since many pumps are on timers, a person may be near or in contact with a pump thinking it is not in operation when it may suddenly start up without warning. Such occurrences have caused serious injuries in the past. In addition many workers' injuries have resulted from fatigue during the long and strenuous process of putting the pump in the casing or removing it for maintenance.
Other types of pumps, usually more expensive, may be used. For very high production volumes, such as are encountered in certain water flood, secondary recovery operations, submersible electric motors of up to 50 feet in length (at one horsepower per foot) are used to drive multi-stage axial-centrifugal hybrid pumps of up to 600 stages. Each stage is a rotor and stator combination about one inch in length. For moderate production volumes at extreme depths hydraulic piston driven axial stroke pumps are used. These pumps have very complex valving reversing the direction of the piston every stroke and they are powered by triplex pumps on the surface of the earth. In these cases either wires or rigid hydraulic tubing must be placed in the hole which complicates the installation appreciably.